1. Field of the Invention
The invention relates to an ink ejecting printer head having at least one wall defining an ink chamber, the wall being formed of a piezoelectric ceramics plate.
2. Description of Related Art
As an ink ejecting printer head using a piezoelectric ceramics plate, there has conventionally been proposed a drop-on-demand type of ink ejecting printer head. In this type of ink ejecting printer head, the volume of a groove formed in a piezoelectric ceramics plate is changed by deforming the piezoelectric ceramics plate. When the volume is decreased, ink contained in the groove is expelled in the form of droplets from a nozzle, whereas when the volume is increased, additional ink is introduced from an ink supply passage into the groove. A plurality of such nozzles are arranged in a neighboring relationship to each other, and the ink droplets are expelled from desired ones of the nozzles according to desired print data to thereby form desired characters or images on a sheet of paper opposed to the nozzles.
This kind of ink ejecting printer head is disclosed in U.S. Pat. Nos. 4,879,568, 4,887,100, 4,992,808, 5,003,679, 5,028,936 and 5,016,028, for example. FIGS. 5 to 8 schematically show such a conventional ink ejecting printer head.
The structure of such a conventional ink ejecting printer head will now be described with reference to FIG. 5 showing a cross section thereof. The ink ejecting printer head comprises a piezoelectric ceramics plate 1 and a cover plate 2. The piezoelectric ceramics plate 1 has a plurality of grooves 12 and is polarized in a direction depicted by an arrow 4. The cover plate 2 is formed of a ceramics material or a resin material. The piezoelectric ceramics plate 1 and the cover plate 2 are bonded together by an adhesive layer 3 formed of an epoxy adhesive, for example, whereby the plural grooves 12 are formed as a plurality of ink channels. Each ink channel is rectangular in cross section and is elongated over the length of the piezoelectric ceramics plate 1. A plurality of side walls 11 defining the ink channels extend over the length thereof. The adhesive layer 3 is formed on the upper surface of each side wall 11. A pair of metal electrodes 13 for applying a driving electric field are formed on the opposed side surfaces of each groove 12 at an upper half portion thereof. A protective film 20 is formed so as to cover each metal electrode 13. All of the ink channels are filled with ink.
The operation of the ink ejecting printer head shown in FIG. 5 will be described with reference to FIG. 6. When the groove 12B, as an exemplary one of the grooves 12, is selected according to desired print data, a positive driving voltage is rapidly applied to the metal electrodes 13E and 13F formed on the inside of the groove 12B, and the metal electrodes 13D and 13G formed on the outside of the groove 12B are grounded. As a result, a driving electric field having a direction 14B is generated in the side wall 11B, and a driving electric field having a direction 14C is generated in the side wall 11C.
As the directions 14B and 14C of the driving electric fields are perpendicular to the direction 4 of polarization of the piezoelectric ceramics plate 1, the side walls 11B and 11C are rapidly deformed inwardly of the groove 12B by a piezoelectric thickness shear effect. This deformation of the side walls lib and 11C reduces the volume of the groove 12B to rapidly increase the pressure of the ink contained in the groove 12B and thereby generates a pressure wave. As a result, the ink droplets are expelled from a nozzle 32 (refer to FIG. 7) communicating with the groove 12B. Further, when the application of the driving voltage is gradually stopped, the side walls 11B and 11C gradually restore to their original positions before deformation, and the pressure of the ink contained in the groove 12B is therefore gradually decreased. As a result, additional ink is supplied from an ink inlet hole 21 (refer to FIG. 7) through a manifold 22 (refer to FIG. 7) into the groove 12B.
In an actual product, however, a driving voltage may be applied in a direction reverse to the above to supply the ink into the groove 12B before expelling the ink, and thereafter the application of the driving voltage may be rapidly stopped to return the side walls 11B and 11C to the original positions and thereby expel the ink.
The construction and a manufacturing method for the ink ejecting printer head shown in FIG. 5 will now be described with reference to FIG. 7 showing a perspective view thereof. The grooves 12 are formed in the piezoelectric ceramics plate 1 by cutting with use of a thin, disk-shaped diamond blade. All of the grooves 12 are parallel and have the same depth over almost the entire length of the piezoelectric ceramics plate 1. The depth of each groove 12 is gradually reduced as it approaches a rear end surface 15 of the piezoelectric ceramics plate 1 to form a shallow groove 16 near the rear end surface 15. Thereafter, the metal electrodes 13 are formed on the side walls 11 by a known technique, such as sputtering. Then, the protective films 20 covering the metal electrodes 13 are formed using a dry or wet process.
On the other hand, the ink inlet hole 21 and the manifold 22 are formed in the cover plate 2 made of a ceramics or a resin material by grinding or cutting. Then, the lower surface of the cover plate 2, in which the manifold 22 is formed, is bonded to the upper surface of the piezoelectric ceramics plate 1, in which the grooves 12 are formed, by means of an epoxy adhesive or the like. Then, a nozzle plate 31, having the nozzles 32 arranged at positions corresponding to the front end positions of the grooves 12, is bonded to the front end surface of the assembly of the piezoelectric ceramics plate 1 and the cover plate 2. A substrate 41 having a plurality of conductor film patterns 42 arranged at the positions corresponding to the rear end positions of the grooves 12 is bonded to the lower surface of the piezoelectric ceramics plate 1 on the opposite side of the cover plate 2 by means of an epoxy adhesive or the like. Each conductor film pattern 42 is connected by wire bonding through a conductor wire 43 to the metal electrode 13 formed on the bottom surface of the shallow groove 16 contiguous to the corresponding groove 12.
The control unit, for controlling the ink ejecting printer head shown in FIG. 5, will be described with reference to FIG. 8 which is a block diagram of the control section. The conductor film patterns 42 formed on the substrate 41 are individually connected to an LSI chip 51. Also connected to the LSI chip 51 are a clock line 52, a data line 53, a voltage line 54, and a ground line 55. The LSI chip 51 determines which nozzle 32 the ink droplets are to be expelled from according to data appearing on the data line 53 on the basis of continuous clock pulses supplied from the clock line 52. Then, according to the result of determination, the LSI chip 51 applies a voltage V of the voltage line 54 to the conductor film pattern 42 connected to the metal electrode 13 in the groove 12 to be driven. Further, the LSI chip 51 applies the zero voltage of the ground line 55 to the other conductor film patterns 42 connected to the metal electrodes 13 in the grooves 12 not to be driven.
In the ink ejecting printer head having the structure described above, it has conventionally been considered that an inactive, inorganic passive film, for example an alternately laminated film of silicon nitride (SiNx) and silicon oxynitride (SiON), is preferable for the protective film 20 that is provided for the purpose of insulating and protecting the metal electrode 13 or for preventing corrosion of the metal electrode 13 itself.
However, the use of such an inorganic material for the protective film 20 for insulating and protecting the metal electrode 13 in the ink ejecting printer head causes a problem because the metal electrode 13 cannot be perfectly protected as the protective film 20 cannot be formed on a recessed portion due to a shadow effect of unevenness peculiar to the piezoelectric ceramics plate 1 as a bed or unevenness of the metal electrode 13 affected by the unevenness of the piezoelectric ceramics plate 1. Further, a surface roughness Ra of the protective film 20 has an influence upon variations of ink droplets to be expelled and upon an average volume of the ink droplets. If the surface roughness Ra is large, the segregation of ink components occurs to increase the variations of the ink droplets and decrease the average volume of the ink droplets, thus deteriorating the ejecting characteristics of the ink ejecting printer head. Moreover, when a segregated substance of ink components comes near the nozzle, an ink expelling direction may be changed or the expelling of ink may be hindered to cause an expulsion defect.